Offerings to the God of Sparks

OFFERINGS TO THE GOD OF SPARKS

At the start of most motorcycle rallies there is a select groupof riders trying to coax a spark from their machine’s reluctantmagneto. Having been there a few times myself I empathisedwith them. The endless kicking, running and bumping, or beingpushed, resulting in a couple of tantalising ignitions, whilst thepints of sweat pour down the inside of your waterproofs, can testthe patience of anybody. There are also more subtle problems thatmight indicate a weak spark; the bike feels like it is running outof petrol (but isn’t), misfires on wide throttle openings, hesitateson sharp throttle openings or refuses to start when hot.

There are four main types of motorcycle magnetos:

fixed magnet, fixed HT coil and switched magnetic field (eg; Dixie or Splitdorf)

fixed magnet and rotating HT coil (eg. BTH or Lucas)

rotating magnet and fixed HT coil (eg. Lucas SR or PAL)

rotating magnet and fixed separate HT coil (Energy Transfer Systems)

However they all rely on a coil of wire moving through amagnetic field producing an electrical current, the stronger themagnetic field or the faster the coil moves through the magneticfield, the bigger the current, the bigger the eventual spark. I willnot go into the detailed theory of how a magneto works, it wouldbore most people.

Motorcycle magnetos produce relatively low voltage and currentsparks (12 to 18 thousand volts) compared to a good coil ignitionsystem (20 to 30 thousand volts) or some modern car systems thatproduce very high voltages and currents (50 thousand volts). Highcurrents and voltages are very useful for burning off spark plugdeposits and igniting weak mixtures.

Motorcycle magnetos have another handicap. When beingkicked over the coils pass through the magnetic field at a relativelyslow speed, producing a low initial current. So to produce aspark at slow speed the magneto must be in good condition. Thefollowing are some hints and tips, learnt from bitter experience,that might help restore the performance of your wilting magneto,or just get you home.

SPARK PLUGS There are two main issues with modern spark plugs, firstly they come with the gap set for modern ignition systems, usually at around 35 thou, and secondly they can come with built in suppressors (see below). The recommended spark plug gaps for most magnetos (when new), were between 20 to 25 thou. The gap can be closed down even further if you are having problems (I once ran a 10 thou spark plug gap for several weeks on a particularly tired magneto). If you are stuck without a feeler gauge the thickness of your thumbnail is a good start point.

SUPPRESSOR CAPSMost motorcycle magnetos were not designed to run with suppressors. Spark plug, or plug cap, suppressors are carbon resistors which reduce voltage and current at the spark plug gap, throw them away.

HT LEAD

Always, always use solid copper HT lead, preferably with the two layers of insulation. Some modern HT leads have a carbon string like core that acts as a suppressor. These can kill a magneto spark stone dead. Have a good look at the routing of the HT cable for rubbing or touching hot bits. I once finished up stranded in the central reservation of a busy British motorway after a HT lead hadmelted on a cylinder head fin.